Cable Stop Barrel Adjusters.

If its a few years old your bike may have them. They allow fine tuning of the gears and sit in sockets on the frame (as seen on this 2011 Specialized Tarmac). They are in a great place to get all sorts of 'stuff' in them and then the cable ends and the barrel adjuster can rust together making adjustments impossible and cable changing a nightmare. You can see the cable end still in the top of this one. When cleaning the bike it takes a couple of minutes to let the tension off the cables, pull the cables out of the adjusters and give them a quick wipe - its worth the effort!!!